1. The Field of the Invention
The present invention relates to systems and methods for regulating a reference current in a current mirror. More particularly, the present invention relates to systems and methods for reducing or eliminating the effect of noise on a reference current drawn by a load such as a photodiode, thereby eliminating the need for an external capacitor.
2. The Relevant Technology
In optical networks, light is transmitted over optical fibers from node to node. Each node usually includes some sort of transceiver, which includes an optical transmitter for transmitting optical signals and an optical receiver for receiving optical signals. The optical transmitter launches an optical signal into the optical fibers of an optical network and the optical signal is ultimately received by an optical receiver that has the ability to identify and process the data in the optical signal.
Detecting an optical signal usually begins when an optical signal exits an optical fiber and impinges a detector in the optical receiver. The detector in many optical receivers is a photodiode such as a PIN photodiode or an avalanche photodiode. A photodiode is a device that converts optical energy to electrical energy by producing a current in response to the optical signal. The current produced by the photodiode is related to the energy level of the optical signal detected by the photodiode.
The photodiode is typically reverse biased and is connected to a current mirror. The photodiode draws a current from the current mirror in response to the power or intensity of the optical signal. The current mirror also measures the current in the photodiode by mirroring the current drawn by the photodiode.
Changes in the current produced by the detector correspond to data in the optical signal and the ability of the optical receiver to successfully interpret the optical signal is often related to the noise. Noise is often defined as electrical or optical energy that is not part of the electrical or optical signal. Noise is present in most optical receivers and the ability of an optical receiver to successfully receive and interpret an optical signal depends on its ability to reject or handle the noise that is present. Noise can seriously impact the operation of the receiver.
For example, a photodiode is typically connected to a power supply through the current mirror. Noise on the power supply can affect the current flowing through the photodiode and adversely affect the ability of the receiver to read the incoming optical signal correctly. In other words, noise in the power supply often passes through the current mirror to the photodiode that receives the optical signal.
One of the ways that noise or the effects of the noise on the power supply are reduced is to connect the photodiode with a filter network that includes a capacitance. At high frequencies, the capacitance has low impedance and can reduce the effects of the noise on the current generated by the photodiode. The current mirror may also have a similar capacitance that is configured to help prevent high frequency noise from reaching the photodiode. In effect, the capacitor decouples the noise.
While capacitors can be formed in integrated circuits, capacitors can consume a significant amount of space. This is one of the reasons that capacitors used to filter or decouple noise on the supply are connected as external capacitors. Even though external capacitors may be less expensive than capacitors integrated into the integrated circuits, external capacitors are components that still involve cost and consume board space.